The Scanning Tunnelling Microscope (STM) technology introduced a new concept in measurements using a probe-type approach to map the surfaces. Atomic Force Microscopy (AFM) is a variation of this technology. The difficulty with AFM and STM is the measurement of trenches in two dimensions. Both require sophisticated multi-connected interferometers to detect the 2-D tip motion.
In the original atomic force microprobe, a very small probe tip with a submicron radius floated across a slowly undulating surface with displacements at the nanometer level to characterize surface terrain of a sample. The tip was vibrated perpendicular to the surface and depending upon the distance from the surface, a change in cantilever tip resonance occurred due to the Van der Waal's force between the surface and the tip. This change in resonance was sensed with a laser heterodyne interferometer. The tip could be maintained either at a fixed height through a feedback loop or the change in resonance could be converted to a height signal. The sensitivity of the change in resonance allowed accurate height measurements at the nanometer level. The tip to sample spacing was in the range of a few tens of nanometers. In this mode of operation, the tip to sample spacing remained constant as the sample was scanned using a piezo-electric scanning stage. The principal drawback of this mode of operation was the limited resolution and accuracy of the measurements in the lateral X-direction due to tip geometry.
G. Binnig, C. F. Quate, and C. Gerber, "Atomic Force Microscope," Physics Review Letters Vol. 56, pp. 930-933 (1986) described an atomic force microscope that was capable of investigating surfaces of insulators on an atomic scale. It was a combination of the principles of the scanning tunnelling microscope and the sylus profilometer. In this system the force between tip and sample was repulsive and four orders of magnitude larger than what could be achieved with AC vibrating techniques applied to the tip.
Later on, Y. Martin, C. C. Williams, and H. K. Wickramasinghe, "Atomic Force Microscope-Force Mapping and Profiling on a Sub-100 Angstrom Scale", Journal of Applied Physics, Vol. 61 (10), pp. 4723-4729 15 May 1987) described a modified atomic force microscope employing a tungsten tip at the end of a wire mounted on a piezoelectric transducer. The transducer vibrated the tip at the resonance frequency of the wire, which acted as a cantilever. A laser heterodyne interferometer accurately measured the amplitude of the AC vibration. The long range force between the tip and the sample is an attractive force of the Van der Waals type. When the tip came very close to the sample, the force of attraction became significant, the tip would stick to the sample, and the force required to pull the tip away from the surface of the sample could be measured. A polarized laser beam was reflected off the wire carrying the tip to monitor the vibration of the lever even when it was excited into vibrations having amplitudes of several hundred angstroms.
U.S. Pat. No. 4,724,318 (Binnig) discloses an atomic force microscope and method for imaging surfaces with atomic resolution. A sharp point is brought so close to the surface of a sample to be investigated that the forces occurring between the atoms at the apex of the point and those at the surface cause a spring-like cantilever to deflect. The cantilever forms one electrode of a tunneling microscope and the other electrode being a sharp tip. The deflection of the cantilever provokes a variation of the tunnel current, and that variation is used to generate a correction signal. Also disclosed is a XYZ-drive which permits a sample to be displaced in X, Y, and Z directions with respect to a stationary point.
U.S. Pat. No. 4,747,698 (Wickramasinghe, et al.) discloses a scanning thermal profiler. Apparatus is provided for investigating surface structures irrespective of the materials involved. Piezo electric drivers move the scanning tip both transversely of, and parallel to, the surface structure. Feedback control assures the proper transverse positioning of the scanning tip and voltages thereby generating a replica of the surface structure to be investigated. Also disclosed is that in any suitable manner, the sample and the scanning tip can be moved relative to each other in three dimensions such as having three piezo drives. Piezo drives operate in the lateral dimensions X and Y, and can also adjust the relative positions of the sample and scanning tip in the Z dimension. Alternatively, the probe may be fixed and the sample moved relative to the scanning tip.
U.S. Pat. No. 4,806,755 (Duerig, et al.) discloses a micromechanical atomic force sensor head. The micromechanical sensor head is designed to measure forces down to 10.sup.-13 N. It comprises a common base from which a cantilever beam and a beam member extend in parallel. The cantilever beam carries a sharply pointed tip of a hard material. In contrast to the scanning tunneling microscope where the tip head had to be electrically conducting the tip, the tip may now be made of any solid material, conducting or nonconducting. The distance between tip and surface typically, will be on the order of one tenth of a nanometer or one angstrom.
U.S. Pat. No. 4,883,959 (Hosoki et al.) for "Scanning Surface Microscope Using a Micro-Balance Device for Holding a Probe-tip" describes a probe tip which can detect a Van der Waals' force. The probe tip is placed on a balance bar which is part of a micro-balance apparatus. A magnetic member controls the magnetic field to control the equilibrium of the micro-balance apparatus. The atomic forces on the probe cause the micro-balance to be displaced by the negative forces between the probe tip and the surface of the sample. It should be noted that Hosoki et al. employed a negative force while a positive force was employed in the Martin et al system.
"Microprobe-Based CD Measurement Tool," IBM Technical Disclosure Bulletin, Vol. 32, No. 7, page 168 (Dec. 1989) describes "a metrology tool which utilizes the atomic force microprobe (AFM) . . . as a surface sensor and is specifically designed for measuring trench depth and width . . . " The system incorporates a two-dimensional length measurement system, such as a two-axis laser interferometer in addition to a two-dimensional laser heterodyne system which detects the change in resonance of the vibrating probe tip as it approaches the surface. The probe tip may be vibrated in either the horizontal or vertical direction depending upon which surface is being approached. The mean position of the probe tip is held stationary while the wafer or other part being measured is moved parallel or perpendicular to its surface and the displacement measured. The article described use of a three-point probe tip with well-defined sensor points for detecting the bottom, right and left edges of the trench. The probe tip is lowered into the trench using robotic motions. The tip is then moved from left to right at a specific height above the bottom of the trench. Trench widths are thereby measured as a function of height, and thus, edge slope can be determined. When the tip dimensions are known, accurate measurements of trench dimensions can be made. In addition, this system provides accurate control of tip position, thereby preventing accidental tip damage.
These above-mentioned techniques, as well as all related methods have the resolving power to map molecular surfaces as long as the topography is shallow. For trenches and complex topography, these approaches are limited due to the tip convolution effects, at the slopes of the trench and/or linewidth to be measured.
The current tips, such as the parabolic tungsten tips have a tip radius of approximately 0.1 micrometers, but 0.01 micrometers and smaller tips have been fabricated in other materials using electron beam deposition techniques. See related U.S. patent applications Ser. No. 07/568,451 and Ser. No. 07/619,378, which now allow the measurement of deep and narrow trenches.
Furthermore, the current SFM technology is limited to a single axis probing and the two-axis probing requires the use of external laser interferometric sensing for the positional information. On the other hand, this invention provides a three-dimensional mapping technique, whereby the linewidth or trench surface is first obtained from the optical profile resulting from the optical metrology technique and which allows one to rapidly profile the surface with a SFM to higher nanometer definitions.
The Scanning Force Microscope (SFM) of this invention can be used for CD (Critical Dimension) linewidth and overlay measurements for sub-half-micrometer feature dimensions, such as 0.35 micrometers and below. The tip geometry is not the only problem with the previous instruments. From a metrology point of view, one needs independent lateral (X) and height (Z) dimensional measurements (at any height). Even by adding a vibration and interferometric sensing system for the change in resonance in the X-direction, the mode of operation described earlier has a second problem, i.e., the tip goes in and out of the X-direction force range in scanning across a line or trough with steep edge slopes.